1. Field of the Invention
My invention relates to the field of key generation, and more specifically to generation of multiple independent key streams from a single underlying key stream.
2. Description of the Prior Art
Key generators, devices which create a seemingly random binary bit sequence, are used in communications for many useful purposes. Examples of common uses include message encryption, anti-jam protection, and spread spectrum communications. When multiple channels are involved, it is common to provide an independent key stream for each channel by simply including a plurality of key generators--one for each channel. Obvious drawbacks of this approach include the increased quantity of items required and the associated problems of synchronizing multiple equipments. A simpler method found in the prior art is to provide the same key stream to each channel, or to provide a different time-delayed offset of the same key stream to each channel. The close mathematical relationship of the individual streams, however, makes analysis of the transmitted signals relatively straightforward with the result that encrypted messages may be broken and anti-jam measures may be defeated.
Still another known method of producing multiple key streams from a single source is to create a single high speed key stream stepping at a rate equal to m.times.n, where m is the number of channels and n is the bit rate required per channel. Each channel may utilize every m.sup.th bit of the stream in a time division multiplex format to develop its own independent n-bit stream. This procedure is limited to the extent that it allows a maximum of m channels per device.
There is a need for an apparatus capable of generating multiple mathematically independent key streams, but having reduced cost, complexity and weight over the structures in the prior art. More specifically, there is a need for an apparatus capable of creating 2.sup.m such key streams, varying numbers of which may be in use at any given time, each key able to provide a high degree of cryptographic security.